Cholesterol depletion with (2-Hydroxypropyl)- β-Cyclodextrin modifies the gating of membrane electroporation-induced inward current in pituitary tumor GH ₃ cells: Experimental and analytical studies

The effects of (2-hydroxypropyl)- β-cyclodextrin (HPβCD), a cyclic oligomer, on membrane electroporation-induced inward current (I _MEP ) in pituitary tumor (GH ₃ ) cells were experimentally and analytically characterized. Depletion of membrane cholesterol by exposing cells to HPβCD (2 mM) increased the activation time constant of delayed rectifier K ⁺ current. Such maneuver resulted in a significant reduction of I _MEP density. 2,2'-Azo-bis(2-amidinopropane) dihydrochloride (AAPH), an initiator of free radicals, increased the magnitude of I _MEP . AAPH-stimulated I _MEP was not reversed by the blockers of Ca ²⁺ -activated K ⁺ channels, but by LaCl ₃ or MnCl ₂ . However, in HPβCD-treated cells, the ability of AAPH to enhance I _MEP was abolished. Under such maneuver, the gating charge of I _MEP activation was increased by 2 fold, along with a hyperpolarized shift of the activation curve by 30 mV. No change in single-channel conductance of MEP-induced channels during cell exposure to HPβCD was demonstrated. The energy change of I _MEP in untreated and HPβCD-treated cells was estimated to be -17.7 and -44.8 kJ/mol, respectively, and the perturbation of free energy following HPβCD treatment was -27.1 kJ/mol. Based on an MEP model, cell exposure to HPβCD increased the edge energy of the electropore size. By use of a two barrier-one site barrier model, HPβCD treatment can increase both the peak height and well depth of the barrier profile. Taken together, depletion of membrane cholesterol by HPβCD can elevate the edge energy of pore formation, thereby decreasing the I _MEP magnitude. The channel-suppressing properties during membrane cholesterol depletion with HPβCD might thus contribute to the underlying mechanisms by which such maneuver alters neuronal or neuroendocrine function.